SEMPILL or SEMPLE, WILLIAM (1546–1633), soldier of fortune and political agent, born in 1546, was a cadet of the noble family of Sempill long seated in Renfrewshire. His exact place in the family tree has been variously stated. His name does not occur in Douglas's ‘Peerage.’ Conn calls him ‘frater baronis,’ which he certainly was not. Other contemporary writers make him the bastard son of the third or uncle of the fourth baron (Colville, Letters, ed. D. Laing, p. 329). Father Hugh Sempill [q. v.], who was undoubtedly his brother's son, describes himself as ‘Craigbaitæus,’ the Sempills of Craigbait or Craigbet being a branch of the family descended from David, a younger brother of the third, or the ‘great’ lord Sempill.

In his youth Sempill was for some time attached to the court of Mary Stuart. He subsequently joined a Scottish regiment under Colonel William Stewart, in the service of the Prince of Orange, and on 25 March 1582 he took the command of a company of Scots in the strongly fortified garrison of Liere, near Antwerp. Here, according to one account, smarting under injuries from Colonel Stewart, and under insults which he had received from the governor of the town, who had threatened to hang him for complaining of the sufferings of the Scottish soldiers (for they had been ten weeks without pay or food, and were compelled to live upon roots), Captain Sempill in revenge resolved to betray the garrison into the hands of the Prince of Parma (W. Herle to Burghley, Hatfield MSS. ii. 511). According to the Jesuit historian Strada, Sempill obtained a secret interview with Parma at Poperinghee, and declared to him that he had purchased his captaincy at Liere only in order to deliver up the place to the Spaniards, and that if he should succeed in this he should ask for no other reward than his own satisfaction in the event. Parma accordingly placed Sempill in communication with Matthew Corvino, an old and experienced soldier, with whom the plan was arranged. On the night of 1 Aug. 1582 Sempill obtained permission on some pretext to make a sortie, and was given thirty Scots and seven States soldiers for the purpose. He then effected a junction with the troops of Corvino, and early in the morning of the 2nd returned to Liere, where by a preconcerted arrangement with his brother, who was serving as a lieutenant in the same garrison, the gates were opened, and after a brief struggle, during which Sempill distinguished himself by slaying the gatekeeper and officer of the watch, the Dutch forces were overpowered and the Spaniards took possession of the town. The moral effects of Sempill's action were considerable, for though Liere was not a large place, it was, on account of its strength and position, regarded as ‘the bulwark of Antwerp and the key of Brabant;’ and the betrayal of Bruges in the following year by Colonel Boyd was probably prompted by his countryman's example. After a short visit to Parma at Namur, Sempill was now (1582) sent into Spain with a strong recommendation to the king, who, says Strada, handsomely rewarded him. In November 1587 Philip despatched him to Bernardino de Mendoza then at Paris, warning the ambassador to be cautious in dealing with him, as, in spite of his apparent zeal, he was nevertheless ‘very Scotch.’ Mendoza, however, was able to report to the king that he found Sempill more trustworthy than most Scotsmen of either sword or gown, and the colonel (as he was now called) was in consequence busily employed in the secret negotiations then being carried on with the catholic nobles of Scotland in view of the projected invasion of England. It was supposed by George Conn [q. v.] that Sempill was also entrusted with a mission to James himself, in the hope of bringing about a marriage of the Scottish king with the infanta of Spain.

Sempill landed at Leith early in August 1588, when he was immediately apprehended by Sir John Carmichael by the king's order. The Earl of Huntly contrived to release him, but James had him again captured and imprisoned in Edinburgh. Once more, by an expenditure of four hundred crowns on the part of Robert Bruce (if this spy and conspirator is to be trusted) and with the aid of Huntly and Lady Ross, a daughter of Lord Sempill, the colonel effected an escape of which a romantic account is given by Father Forbes-Leith in his ‘Narratives of Scottish Catholics’ (p. 368). The privy council now (Aug. 20) issued an order ‘against resetting William Semple, who had come on a pretended mission from the Prince of Parma and had been trafficking treasonably with His Majesty's subjects.’ Before leaving Scotland for the Low Countries Sempill made arrangements for carrying on a secret correspondence with his friends; and in February of the following year his servant, Pringle, was captured in England with a packet of treasonable letters, directed by Huntly, Errol, and others to Parma and the king of Spain. Pringle confessed to Walsingham that he had been sent over from Flanders by Sempill six weeks before. The colonel's name frequently reappears in the state papers of 1593–4 in connection with the Spanish intrigues and military enterprises of that time, but he does not seem to have again visited Scotland.

In 1593 he married in Spain Doña Maria de Ledesma, widow of Don Juan Perez de Alizaga, and daughter of Don Juan de Ledesma, member of the council of India. In 1598 Robert, the fourth lord Sempill, who had been appointed Scottish ambassador at Madrid, was instructed by James to sound the intentions of Philip III with regard to the succession to the English crown. Lord Sempill in his correspondence frequently mentions the assistance he had received from ‘the crunal my cusing,’ while the colonel himself wrote to James (12 Oct. 1598) of ‘the lang intension that I haif haid to die in my cuntre in yor Maties service’ (Miscellaneous Papers, Maitland Club, p. 173). Sempill lived to a great age, occupying at the Spanish court the office of ‘gentleman of the mouth’ to the king, and busying himself with the affairs of the catholic missionaries in Scotland to whose support he liberally contributed, as is shown by the letter of Father Archangel Leslie, addressed to the colonel 20 June 1630, printed in the ‘Historical Records of the Family of Leslie’ (vol. iii. p. 421).

In 1613 Philip III had granted to Sempill the house of Jacomotrezo in Madrid as an equivalent of the sums due to him in arrears of salaries and pensions. This house he designed and endowed as a college for the education of catholic missionaries who were to be drawn from the gentry of Scotland, and by preference from members of his own family. The government of the college was to be in the hands of the Jesuit fathers. The original deed of foundation and endowment, dated 10 May 1623, was printed by the Maitland Club (Miscellaneous Papers), together with a translation of the colonel's testament, dated 20 Feb. 1633. He died in this house on 1 March 1633, at the age of eighty-seven. His wife survived him, dying on 10 Sept. 1646.